The present invention relates to a communications terminal and a method of operating a communications terminal.
The inflexibility of frequency division multiple access and time division multiple access techniques has resulted in the development of new systems based on the uncoordinated spread spectrum concept. In these new systems, the bits of slow speed data traffic from each subscriber are multiplied by a high chip rate spreading code, forcing each low bit rate (narrowband) data signal to fill a wide channel bandwidth. Many subscribers can then be accessed by allocating a unique, orthogonal spreading code to each. This constitutes a code division multiple access (CDMA) system. In the receiving terminal, the desired signal is detected by correlation against a local reference code which is identical to the particular spread spectrum code employed prior to transmission.
Rapid initial code acquisition and re-acquisition is crucial in CDMA communications. DOPPLER or local oscillator offsets can lead to frequency uncertainty which make this task particularly difficult. Existing strategies for code acquisition are single and multiple dwell detectors, matched filters, sequential detection and parallel detectors. In the presence of frequency uncertainty, the most common approach is to sequentially search all code phases over the range of anticipated frequency offsets. This brute force approach is laborious and can lead to large acquisition times.
Current second generation mobile communication systems cannot provide sufficient capacity to support the future demands of increased subscribers and higher data rates for multimedia communications. Third generation systems will be required to provide multiple access schemes which are capable of flexible data rates and variable services. However, it will considerably aid the acceptance of third generation systems if existing standards, infrastructures and components can be reused or reconfigured.
One common way of acquiring a direct sequence spread spectrum (DS-SS) signal is through the use of an energy detector at the output of the despreader. This approach works by tuning the code phase and frequency offset of a complex matched filter over the range of possible phase and frequency offsets anticipated in the system. When the phase of the desired spreading sequence (usually measured in terms of code chips) and local oscillator frequency offset are within specified limits, the detector will produce an output which exceeds some threshold and the system will register the presence of the desired user. This initial acquisition will then trigger a verification loop which confirms the presence of the desired code sequence and subsequently a tracking loop which attempts to continuously maintain close alignment between the two code sequences in order to track any fluctuations. If the desired spreading sequence phase and frequency offset are not within the limits, the output of the detector will not exceed the threshold, and the search for initial acquisition will continue.
The number of possible search bins will be determined by the product of the number of possible code phase offsets (typically a half chip search is employed over the entire sequence), and the total range of possible Doppler offsets. The width of each Doppler bin is in turn determined by the frequency response of the complex correlator, thus yielding a 2-dimensional search plane as depicted in FIG. 1. The acquisition cells system sequentially searches this grid by aligning the reference code phase and frequency offset to the centre of each cell. The time required in order to obtain an initial acquisition will therefore depend directly on the number of cells in the search region. For systems with long spreading codes, which might experience large Doppler offsets, this acquisition time may prove prohibitively large.
We will consider, for example, a system with random codes of length 200, in which the range of possible Doppler shifts is +/−32 kHz. The normalised frequency response of a complex matched filter (MF) is a function of the Doppler frequency offset Δf, as given below.                                                     H            ⁡                          (                              Δ                ⁢                                                                   ⁢                f                            )                                                =                              1            M                    ⁢                                    sin              ⁡                              (                                  M                  ⁢                                                                           ⁢                  πΔ                  ⁢                                                                           ⁢                                      fT                    c                                                  )                                                    sin              ⁡                              (                                  πΔ                  ⁢                                                                           ⁢                                      fT                    c                                                  )                                                                        (        1        )            
In (1), M is the length of the spreading code, Δf is the Doppler frequency offset and Tc is the chip duration. FIG. 2 depicts the frequency response of a complex MF for a data rate of 8 kHz (M=200, Tc=625 ns). We see that the 3 dB bandwidth of the complex MF with these parameters is around 4 kHz. This will result in a total of (200×2)×(64/4)=6400 cells which the energy detector must search. It has therefore been proposed, for example in Sust et al, “Rapid Acquisition Concepts for Voice Activated CDMA Communication” IEEE Globecom 90, pp 1820-1828, and in Stirling-Gallacher et al “A Fast Acquisition Technique for a Direct Spread Signal in the Presence of a Large Doppler Shift” IEEE ISSSTA 96, pp 156-160, to introduce a FFT based improvement to the energy detector which will reduce the number of possible cells by evaluating a reduced search space.
By re-examining FIG. 1, it would be possible to substantially reduce the acquisition time, if it were possible to search all possible code Doppler cells simultaneously. By employing a FFT block as part of the acquisition system it would be possible to perform this form of search procedure. A block diagram of such a FFT enhanced acquisition system is shown in FIG. 3.
The system consists of P complex matched filter correlators 1, each of length X, such that the product X×P equals the code length M. The first correlator will contain the first X chips of the spreading sequence, the second will contain the next X chips, and so on through the P correlators. The outputs of the P correlators are therefore partial correlation results. These partial results are then passed to a N-point FFT 2, where N=P. The processing gain of this receiver is the same as the original energy detector, however if the correlator length X is chosen correctly, the addition of FFT processing allows the simultaneous search of all possible code Doppler shifts.
A specific example should illustrate this more effectively. Continuing with the previous example, where M=200 and Tc =625 ns, in order to increase the bandwidth of the partial correlators to beyond +/−32 kHz, their length is decreased to X=25. Therefore the number of partial correlators is P=8. If N=P i.e. a 8-point FFT is selected, FIG. 4 depicts the output of a maximum signal selector superimposed on all the FFT bin outputs. The bandwidth of the FFT processor has increased, and is much improved over the standard complex correlator. To further increase the bandwidth, the length of the individual correlators should be reduced and the total number of correlators increased correspondingly.
This FFT enhancement will allow all possible code Doppler offsets to be searched simultaneously. However, a scalloping loss can be observed when the Doppler offset falls between two bins of the FFT. This will result in a reduced probability of detection for signals with these Doppler offsets, as compared to signals which occur in the centre of any given bin.